(1) Field of the Invention
The present invention relates to an improvement in the cap for a writing instrument and, more particularly, to a cap for a writing instrument which has an ink tank for directly storing an ink without allowing it to be absorbed by a sliver or the like, and is provided with an ink reservoir for temporarily reserving the ink pushed out from the ink tank when air in the ink tank expands due to a temperature rise or the like, so that the ink will not drop from a point assembly or vent, a means for fixing a seal cylinder made of an elastic resin or rubber into a cap to seal the point assembly and vent of a writing instrument, and a cap for a writing instrument having an inner cap.
(2) Description of the Prior Art
In a conventional direct ink storage type writing instrument, the seal cylinder is movable in the axial direction so that the volume of its interior does not change when the cap is attached or detached, and the opening edge portion of the seal cylinder is hermetically sealed by abutting it against the conical surface of the mouth piece of barrel cylinder of the writing instrument with an appropriate pressure.
In a so-called sliver-type writing instrument, the cap is attached to the barrel cylinder to hermetically seal the point assembly and the vent. In order to ensure sealing, when the cap is attached to the barrel cylinder, the barrel cylinder and the seal portion of the cap are tightly fitted with each other immediately before this cap attaching operation is ended. While the cap is being attached, the internal space in the cap reduces. As a result, when the cap is being attached, the interior of the cap is pressurized. Inversely, when the cap is to be detached, while the tight fitting portion of the seal portion is being removed, the space in the cap increases, and accordingly its pressure is reduced. However, in the sliver-type writing element, since the interior of the cap and the rear half of the barrel cylinder where the sliver is stored are spatially continuous through a vent in the barrel cylinder, for the writing instrument the interior pressure is either increased or reduced as the entire portion. The pressure in the ink reservoir and that in the point assembly are equal, and when the cap is attached or detached, the ink will not flow out from the point assembly or vent. In the direct reservoir type writing instrument, meanwhile, a volume change in the seal cylinder is prevented during attachment and detachment of the cap due to the following reason. That is to say, assume that upon attachment or detachment of the cap, the pressure in the seal cylinder increases or reduces. If the interior of the seal cylinder is pressurized, air in the seal cylinder is compressed. Air corresponding to the compressed air flows out from the vent and enters the ink tank as bubbles through the ink reservoir. The ink overflows from the ink reservoir in an amount corresponding to the bubbles that have entered, and is reserved in the ink reservoir. Inversely, when the cap is removed from the barrel cylinder, the pressure of air in the seal cylinder is reduced, in the opposite manner to the phenomenon described above. Therefore, the ink in the ink tank is drawn out by vacuum and is reserved in the ink reservoir, or is drawn out from the point assembly by suction. In this manner, every time the cap is attached or detached, the reserved ink amount of the ink reservoir increases to finally exceed the reservoir capacity. Then, the ink flows out from the vent, or leaks from the point assembly. In order to prevent this phenomenon, the conventional direct ink reservoir type writing instrument employs the following structure.
In the cap of the conventional direct ink reservoir type writing instrument, the seal cylinder is movable in the axial direction in order to hermetically seal the comparatively hard seal cylinder, made of polypropylene or polyethylene, without increasing or reducing the pressure in the seal cylinder when the cap is attached or detached, so that the opening edge portion of the seal cylinder comes into contact with the conical surface of the mouth piece of the barrel cylinder of the writing instrument. This makes the structure being complicated and the number of components increased. This results in increase in product unit cost and the complicated assembly further lead to increase in assembly cost.
When the seal cylinder made of an elastic resin or rubber is to be fixed in the cap of the conventional writing instrument, the seal cylinder is pressed into and fitted with an inner cylinder arranged in the cap or with a plurality of ribs the diameter of the inscribed circle of which is slightly smaller than the outer diameter of the seal cylinder.
Furthermore, with a means for fixing the seal cylinder made of the elastic resin or rubber into the cap of the conventional writing instrument, when the cap is mounted on the writing instrument main body, the opening inner edge and flange portion of the seal cylinder come into tight contact with a portion around the vent in the front portion of the barrel cylinder, to close the vent. Nevertheless, since the seal cylinder is made of the elastic resin or rubber, an adhesion force acts also on the barrel cylinder due to the self-tackiness. Accordingly, when the cap is detached, the seal cylinder adheres to the barrel cylinder to come off from the cap.
According to another prior art, a writing instrument is known which has an ink tank in its barrel cylinder and an ink reservoir. The ink tank directly stores the ink. When the ink in the tank is pushed out excessively due to expansion caused by a temperature rise or the like, the ink reservoir temporarily absorbs the excessive ink so it will not be flown out from the point assembly and the like. As a cap that can be attached on and detached from such a writing element, one having an inner cap that can seal the point assembly and an air exchange hole formed in a writing element barrel cylinder near the point assembly is known. However, the above-described cap structure also has the following drawbacks.
When a cap having such an inner cap is employed, after the point assembly of the writing instrument is inserted in the inner cap, the writing instrument barrel cylinder must be engaged with the cap main body, so that the inner cap reliably seals the point assembly of the writing instrument. Hence, after the point assembly of the writing instrument is inserted in the inner cap and is sealed, it is further pushed into the inner cap. Air in the inner cap is compressed and pressurized. The compressed air enters the ink tank in the writing instrument barrel cylinder through the air exchange hole to excessively push out the ink in the ink tank. The pushed ink is reserved in the ink reservoir. When the cap is to be removed from the writing instrument barrel cylinder, the internal pressure of the inner cap is reduced inversely. Accordingly, the ink is excessively drawn out by suction from the ink tank and is similarly reserved in the ink reservoir. Every time the cap is attached and detached, the ink in the ink reservoir increases. When the ink reservoir becomes full of the ink, the overflowing ink leaks from the air exchange hole.
In order to solve these problems, the present applicant previously developed a cap as disclosed in Japanese Utility Model Publication Hei 6 No.17581. An inner cap is mounted in this cap to be movable in the axial direction while being supported with a coil spring. When the inner cap is pushed by the writing instrument barrel cylinder, it moves while compressing the coil spring, so air in the inner cap will not be compressed by the writing instrument barrel cylinder.
However, use of the coil spring increases the number of components and the number of assembly steps, leading to an increase in cost.